1. Field of the Invention
The present invention relates to a fingerprint input device. In particular, it relates to a fingerprint input device having a function of detecting a blood pulse and an electronic device provided with the same.
2. Description of the Related Art
Conventionally, diagnostic instruments for blood circulation, which detect a second derivative of a plethysmogram (acceleration plethysmogram) from a tip of a finger for determining a condition of peripheral blood circulation, have been known. For example, Japanese Patent Laid-Open No. 2000-23928 specification discloses one of such diagnostic instruments. FIGS. 9A and 9B show a configuration of a blood pulse detector described in the specification. In FIGS. 9A and 9B, the blood pulse detector comprises a light emitter 110 for irradiating a finger F with a light, a light detector 120 for detecting light scattered at the finger F, and a housing 130 for securely positioning the finger F with respect to these components. In FIG. 9A, the light emitter 110 and the light detector 120 are disposed side by side below the finger F. In FIG. 9B, the light emitter 110 and the light detector 120 are disposed with the finger F interposed therebetween.
Here, operations of the arrangements in FIGS. 9A and 9B will be described. In the arrangement shown in FIG. 9A, the light from the light emitter 110 is scattered or diffused in the finger F, and the scattered or diffused light is detected by the light detector 120. In the arrangement shown in FIG. 9B, the light from the light emitter 110 passes through the finger F and then detected by the light detector 120. For both the arrangements in FIGS. 9A and 9B, the light detector 120 produces output indicating a blood volume in capillaries of the finger F reflected therein. Therefore, the time variation in the output of the light detector 120 can be monitored to determine a plethysmogram, which is a time variation in the blood volume. Besides, as is well known, a condition of blood circulation of a patient can be diagnosed based on an acceleration plethysmogram. The acceleration plethysmogram can be determined by differentiating the plethysmogram two times. Therefore, if the plethysmogram obtained with the arrangement in FIG. 9A or 9B is analyzed with an external computing device, the condition of blood circulation can be diagnosed.
Conventionally, furthermore, slim fingerprint input devices have been known. For example, U.S. Pat. No. 5,446,290 discloses one of such fingerprint input devices. FIG. 10 shows essential components of the device described in the U.S. Pat. No. 5,446,290. The fingerprint input device comprises a stack of a planar light source 210, a two-dimensional image sensor 220 and a collected fiber member 230. The planar light source 210 is a thin light source comprising light-emitting diodes (LED) disposed on an end of a light-guide, which is commonly used as a back light source of a liquid crystal display (LCD).
The two-dimensional image sensor 220 comprises pixels two-dimensionally disposed on one surface of a transparent substrate 221. Each pixel comprises a switch element 222 and a photoelectric conversion element 223, and is connected to a wiring 224 for the switch, a wiring 225 for signal readout and a wiring 226 for biasing. The transparent substrate 221 has an opening 227 provided at a region where these wirings and pixels are not included, and the region can transmit light. The collected fiber member 230 is fabricated in such a manner that a lot of optical fibers are fused together, the fused optical fibers are cut into plates, and then the surfaces of the plate are polished. An incident light passes through cores 231 of the collected fiber member 230, and thus, an image formed on one surface of the member is transmitted to the other surface thereof.
Next, an operation of the fingerprint input device will be described. The light from the planar light source 210 passes through the opening 227 in the two-dimensional image sensor 220 and the cores 231 in the collected fiber member 230 in this order, and is applied to a finger (not shown) that is pressed against an upper surface of the collected fiber member 230. Then, the light reflected and scattered at the finger is incident on the cores 231 of the collected fiber member 230 and then detected by the photoelectric conversion elements 223 of the image sensor 220. Electric signals produced in the photoelectric conversion elements 223 are read out through the wirings 225 for signal readout sequentially under the control of control signals applied to the wirings 224 for switching, and the read-out signals are recorded in an external circuit (not shown), whereby a fingerprint image can be obtained.
The conventional diagnostic instrument for blood circulation shown in FIGS. 9A and 9B does not have the function for inputting a fingerprint image. On the other hand, the conventional fingerprint input device does not have the function to detect a blood pulse. If the two functions, the function to detect a blood pulse and the function to input a fingerprint image, are integrated into one device, such a device would have various applications.
To integrate the two capabilities into one device, it may be readily contemplated that the components of the two kinds of devices are arranged side by side in one housing. However, such an arrangement has the following problems.
First, in order to attain high precision in personal identification, it is essential that a fingerprint image contains a lot of characteristic features (end points or branch points of ridges of a fingerprint), each of which is specific to an individual. For this reason, the collected fiber member needs to be positioned at the center of the finger. On the other hand, in order to detect a blood pulse with sufficient sensitivity, the light detector is desirably disposed where the maximum amount of the light scattered or diffused at the finger is obtained. This indicates that both the collected fiber member and the light detector are to be disposed at the center of the finger, which is physically impossible.
Second, components having similar functions, such as the light emitter and planar light source both capable of emitting light, and the light detector and two-dimensional image sensor both capable of photoelectric conversion, are to be redundantly provided. However, this undesirably leads to an increase in size and manufacturing cost of the device.
In this case, the blood pulses may be detected by the photoelectric conversion element of the two-dimensional image sensor so that the separate light detector can be removed. However, with such an arrangement, the acceleration plethysmogram cannot be obtained with high precision. Specifically, in a typical fingerprint input device, the two-dimensional image sensor includes 500 by 500 pixels, and a readout time per pixel is 100 ns, for example. In this case, the time required for the two-dimensional image sensor to read out one image is 25 ms. That is, a sampling of the actual blood pulse is obtained every 25 ms. This time interval is more than a time constant of the derivations for providing the acceleration plethysmogram (10 ms in the Japanese Patent Laid-Open No. 2000-23928, for example), and thus, the acceleration plethysmogram cannot be obtained with high precision.